1. Field of the Invention
The present invention relates generally to gyroscopic reference systems of the type including a plurality of strapped down, force-rebalanced gyroscopic rate sensors for measuring craft rotation rates about primary craft axes, together with a system for computing from such measures aircraft stabilization and attitude data. A typical strapped down system is disclosed in the copending U.S. application Ser. No. 907,228, filed on May 18, 1978 in the names of M. Klemes and D. Duncan and entitled "Strapped Down Attitude and Heading Reference System for Aircraft Employing Skewed Axis Two Degree of Freedom Rate Gyros" and assigned to Sperry Rand Corporation. More specifically, the invention relates to pulse width torque feed back apparatus for supplying precision currents to the gyro torquer and for supplying a corresponding precision pulse count proportional thereto and therefore to aircraft rates for use in the digital computation of the stabilization and attitude data. The apparatus further includes improved rate range switching arrangements.
2. Description of the Prior Art
Strapped down gyroscopic inertial reference apparatus for aircraft and space vehicles is well known to those skilled in the art of gyroscopic aircraft control systems; many rate gyro configurations and control systems based thereon have been described extensively in the literature. In general, such systems include a plurality of rate sensors strapped down to the vehicle for measuring the angular velocity of the vehicle about its primary axes, which measure, along with vehicle acceleration and heading measures, is supplied to a digital computer to provide output data for use in stabilization, control, navigation, or guidance of the aircraft. Since the gyros are strapped to the airframe, the rate sensors are preferably of the force or torque rebalancing type; that is, the gyro is maintained substantially aligned with its support case by feeding the gyro pick-off signal back to the gyro torquer in a manner to maintain the pick-off signal value essentially null, the torquer current so required being a measure of the rate being sensed by the rate gyro. Typical two-degree-of-freedom rate sensors of this type are disclosed in the T. R. Quermann U.S. Pat. No. 3,529,477 for a "Gyroscopic Rotor Suspension", issued Sept. 22, 1970 and in the C. G. Buckley, J. A. Kiedrowsky U.S. patent application Ser. No. 818,486, filed July 25, 1977 for a "Permanent Magnet Torquer for Free Rotor Flexure Suspended Gyroscopes", both inventions being assigned to Sperry Rand Corporation. It will be appreciated that the ultimate output of the gyroscopic rate sensor must be compatible with the requirements of available digital computer techniques.
Prior art arrangements have been used to convert the torque feed back signals of a force rebalanced sensor into a form compatible with digital computation techniques, such as, for example, by using a voltage-to-frequency conversion technique or by using pulse width modulation techniques. The present invention utilizes the latter technique. One prior art pulse width modulation type of torque feed back control for an inertial sensor of which Applicant is aware is disclosed in U.S. Pat. No. 4,062,004 to Roantree et al for "Dual Range, Torque Rebalancing of Inertial Sensor", issued Dec. 6, 1977. The over-all function of the Roantree device is similar to that of the present invention; that is, to derive a pulse count in accordance with the feed back current supplied to the sensor torquer and hence in accordance with the sensed inertial parameter and also to provide for both low range and high ranges of operation.
There is a need for retaining at least the ability of the prior art to provide a precise digital measure of the sensed parameter and also to accomplish the measurement more economically in terms of hardware complexity, reliability, and cost. In the prior art, the quantizing pulses are counted up or down depending upon whether the pulse width modulated square wave produces positive or negative feed back currents for the sensor torquer; that is, the counter counts negatively during the first half cycle of the ramp frequency after the gyro signal is compared with the ramp, or it counts positively during the second half cycle before such comparison. A disadvantage of using this technique is that it requires an up-down counter, which is larger and more expensive than a simple one way counter and requires more complex logic circuits. Also, the prior art does not account for the effects of finite rise and fall times or for any over-shoots of the current supplied to the torquer or in controlling his counter. Accordingly, the counter output may not precisely represent the actual torque applied to the gyro and the resulting rate measure. In the Roantree patent, high-low mode detection and switching is based on the state of a clocked comparator output responsive to the gyro error signal, with respect to high and low rate strobe pulses. Also, since high-low determination is performed every ramp cycle, it necessitates hardware hysteresis or a time delay provided by a pulse delaying counter. Furthermore, the current supplied to the gyro torquer in this high rate mode may not be accurate, as it includes the uncertainties associated with the off-set voltages of an on-state switching device.